Human Rights Watch says more than two-thirds of the people held at the Guantanamo Bay prison are housed in inhumane conditions that could damage their mental health.

The New York-based group reported Tuesday that 185 of the 270 detainees at the U.S. military prison for terrorism suspects are kept in small, isolated cells and have limited human contact.

The report says even though some of the prisoners have not been charged with any crime or have been cleared for release or transfer, they are locked up for 22 hours a day with little or no natural light or fresh air. It says their two-hour recreation time is generally spent alone, often in the middle of the night.

The U.S. military has repeatedly said it treats the prisoners according to international standards.

The report also says the detainees have not been allowed family visits and few have been able to call home. Several are reportedly suffering from depression and anxiety disorders.

Human Rights Watch says military officials at Guantanamo have said they plan to increase prisoners' recreation time and allow detainees to congregate. The group says the prison has not announced a schedule for the changes.

A counterterrorism expert with Human Rights Watch, Jennifer Daskal, warns that the prison conditions are creating damaging psychological effects that will breed hatred and resentment of the United States over the long term.

She says the Guantanamo prison should be reformed and, ultimately, closed.

The United States has long been under international and domestic pressure to close the prison camp, where critics say human rights have been abused. The U.S. government denies allegations of abuse. And President George Bush, has said he, too, wants to see it eventually shut down.

The prison opened in 2002 to detain people suspected of committing terrorism.